With the advancement of portable and cordless electronic instruments, growing expectation has been directed to non-aqueous electrolyte secondary batteries smaller in size, lighter in weight, and higher in energy density. In non-aqueous electrolyte secondary batteries, carbon materials such as graphite are used as a negative electrode active material in practical applications. However, carbon materials have a theoretical capacity density of as low as 372 mAh/g. In order to increase the energy density of non-aqueous electrolyte secondary batteries, an attempt has been made where as the negative electrode active material are used silicon (Si), tin (Sn), germanium (Ge), an oxide thereof, and an alloy thereof which can form alloys with lithium. These materials have a higher theoretical capacity density than carbon materials. In particular, silicon-containing particles such as silicon particles and silicon oxide particles have been widely studied because they are less expensive.
However, when these materials are used as a negative electrode active material and are subjected to repeated charging and discharging, the particles of the negative electrode active material change their volume. This change in volume causes the active material particles to be collapsed into fine particles, thereby lowering the conductivity among the particles. As a result, satisfactory charge-discharge cycle characteristics (hereinafter, cycle characteristics) are not attained.
Japanese Patent Unexamined Publication No. 2004-349056, for example, discloses a technology using composite particles (composite negative electrode active material) produced as follows: active material particles containing metal or semimetal that can form lithium alloys are used as the cores (active material cores), and a plurality of carbon fibers are bound to each of the active material cores. It has been reported that this structure can ensure the conductivity even if the active material particles change in volume, thereby maintaining sufficient cycle characteristics. Negative electrodes having high capacity and high functionality are considered to be structured, for example, by using a technology for adequately combining binders that are disclosed in Japanese Patent Unexamined Publication No. H11-354126, in addition to the former technology.
However, only by mixing a plurality kinds of binders, it is difficult to avoid such an accident that a mixture layer containing the composite negative electrode active material is peeled off from a negative electrode current corrector by an action of stress generated when the composite negative electrode active material expands and contracts. It is also difficult to prevent the composite negative electrode active material from dropping off from the mixture layer. This is probably because the surface physical property of silicon-containing particles is different from that of carbon fibers. Negative electrodes having a mixture layer with sufficient binding force are difficult to be produced only by mixing a plurality kinds of binders without considering these surface physical properties.